Habu - I assumed you meant a rifled gun, not a smoothbore. If rifled, a bit more care and you'd have a wonderful big game hunting rifle - double rifle - best for that purpose until the repeating guns replaced them near the end of the 19th century. Of course, that replacement was only due to cost of a fine double, not game or hunting worthiness. Today, many still prefer the double rifle.
In the early to mid 1800's, many preferred the double ML rifle as well. I've a book with an old 'picture' of some 'hunters' at their cabin, one of whom has a double ML rifle with a sword bayonette attached, of all things. Of course, the sights are attached to the centre rib as they should be. The sight base (quarter or shorter) rib could be attached with screws, of course, but much care is needed when moving the rear sight in it's dovetail. The screws can easily shear off at the rib. I know this for fact.
For a smoothbored 'camp or close range double big game gun, one of the Navy Arms 10 or 12 bore 'magnum' muzzleloading shotguns, if reduced to 24" or 26" in the barrels would make a terrific short range big game smoothrifle. Many or most of the modern-made ML double barreled shotguns do not have enough drop in the stock to do this.
To shoot strong with both shot and ball, Forsyth recommended 30" of barrel. For ball alone, 22" to 26" was all that was needed. The large bore seemingly gave good velocity to the ball with normal powder charges and didn't need a long tube to accomplish this task. We see this in bore sizes as small as .58 in Lyman's old book. Note the charge weight per fps from 24" to 32". Not much difference and with some loads, the shorter tube was faster. With today's 2F GOEX powder, my 24" .58 makes 1,308fps compared to 1,077fps for the same charge in 1970 using the GOX of the day.
Keep us posted on this build, Habu.